1. Sea Floor Spreading
Chapter 1-4

2. Mid Ocean Ridges Mapped by sonar in mid 1900’s
Curve like seams on a baseball along the sea floor
Extend across all oceans
Some are completely under water
Some poke through
Iceland

3. How were these ridges formed?
Studied by American geologist Harry Hess in 1960’s
Thought Wegner may have been right
Proposed idea of sea floor spreading

4. Sea Floor Spreading
Sea floor spreads apart at mid ocean ridge as new crust is added.
Molten material erupts from ridge, cools into a strip of new rock
Older rock moves outward as it is replaced by new molten rock
Cause ocean floors to move like conveyor belts carrying continents with them

5. Evidence from Molten Material
Pillow shaped rocks found along mid ocean ridge by the crew of the submarine Alvin.
Rocks can only be formed by molten material that cools quickly under water.

6. Evidence from Magnetic Stripes
Scientists pattern of magnetized “stripes” of rock on ocean floor
Hold record of reversals of Earth’s magnetic field

7. Evidence from Drilling Samples
Drilling ship Glomar Challenger brought up samples from ocean floor that showed rocks further away from ridge were older and closer rocks were younger

8. Why doesn’t the ocean floor keep on getting wider
Part of the ocean floor sinks back into the mantle at deep ocean trenches by the process of subduction

9. Subduction
As new, hot oceanic crust moves away from the mid ocean ridge, it cools down and becomes more dense.
Gravity causes denser crust to sink down into trenches

10. Subduction and the Earth’s Oceans
Ocean floor is renewed about every 200 million years because of seafloor spreading and subduction.
Pacific Ocean is shrinking because it’s numerous trenches are swallowing more crust than is being formed
Atlantic ocean is growing because it has less trenches